Catheters and other endovascular access tools have long been used in the art as medical devices to advance therapeutic agents to an anatomic point of interest for examination, diagnosis, and intervention. Accurate positioning of such interventional devices requires monitoring to ensure that the device is being advanced through the correct structures without causing injury, failing mechanically, and for other reasons known to one skilled in the art.
Methods existing in the art for such monitoring include X-ray visualization, as well as MRI tracking of any component of the device designed to be visible on MRI. Many conventional vascular interventional procedures use X-ray imaging technology in which catheters or other probes are inserted into a vein or artery and navigated to specific locations for diagnostic and therapeutic procedures. For example, over 3,000 trans-septal procedures are performed each year in the United States for left sided radio-frequency ablation therapy and mitral valvoplasty procedures. However, 3-6% of these are complicated by aortic or atrial perforations due to incorrect needle positioning. This relatively high complication rate can in part be attributed to the inability to directly visualize critical endocardial landmarks using a two-dimensional projection x-ray fluoroscopy. Conventional X-ray guided interventions suffer from a number of limitations, including: (1) limited anatomical visualization of the body and blood vessels during the examination, (2) limited ability to obtain a cross-sectional view of the target tissue or blood vessel, (3) inability to characterize important pathologic features of atherosclerotic plaques, (4) limited ability to obtain functional information on the state of the related organ, and (5) exposure of the subject to potentially damaging x-ray radiation.
Many invasive cardiovascular procedures, such as traversing total chronic vascular occlusions, would benefit from using MR guidance to accurately deliver interventional medical devices to target locations because MRI methods have fewer limitations than conventional X-ray techniques. For example, U.S. Pat. No. 6,606,513 to Lardo et al describes a method means for an MR trans-septal needle that can be visible on an MRI, can act as an antenna and receive MRI signals from surrounding subject matter to generate high-resolution images, and can enable real-time active needle tracking during MRI guided trans-septal puncture procedures. Interventional cardiology would also benefit from using MR guidance by exploiting MRI's excellent soft tissue contrast. For example, MRI is able to distinguish between infarcted and healthy myocardium to identify an appropriate location for stem cell delivery [1]. MRI is also able to distinguish between plaque and vessel walls [2-3] which facilitates traversing total chronic occlusions. Furthermore, recent research has demonstrated that a complete electrophysiologic study can now be performed entirely under MRI guidance, including the ability to navigate catheters and characterize the temporal and spatial formation of ventricular radiofrequency ablation lesions in vivo.
Additionally, MRI has been shown to guide mitral valvoplasty procedures. As these two therapies account for 95% of all trans-septal procedures performed, it is clear that the ability to perform safe trans-septal needle puncture under MRI guidance will be of great importance as the field of interventional cardiovascular MRI continues to evolve.
In order to perform such procedures under MR guidance, it is necessary to visualize a cardiac catheter and track it using MRI. Another key requirement in minimally-invasive or non-invasive procedures is to integrate the positioning of instruments, needles, or probes with image guidance to confirm that the trajectory or location is as safe as possible, and to provide images that enhance the ability of the physician to distinguish between tissue types. Placement may require acquisition of static images for planning purposes, either in a prior MRI examination or during the interventional MRI session, or real-time images in arbitrary scan planes during the positioning process. (Daniel et al. SMRM Abstr. 1997; 1928; Bornert et al. SMRM Abstr. 1997; 1925; Dumoulin et al. Mag Reson Med 1993; 29: 411-415; Ackerman et al. SMRM Abstr 1986; 1131; Coutts et al., Magnetic Resonance in Medicine 1998, 40:908-13).
Despite the several distinct advantages MRI has over x-ray fluoroscopy, MRI guided positioning of catheters and other devices has numerous challenges related to imaging artifacts, electromagnetic interference, and the necessity for cardiac and respiratory gating and rapid imaging and display. The invention described herein describes a procedure and required hardware to perform MR guided procedures with active tracking of the tip of the interventional device. Such a procedure may improve imaging applications in a number of interventional MRI guided therapies.
Device tracking techniques using MRI can generally be separated into passive and active tracking methods. Passive tracking is based on device visualization due to magnetic susceptibility artifacts. Magnetic susceptibility is a quantitative measure of a material's tendency to interact with and distort an applied magnetic field. Magnetic susceptibility artifacts are generated by inhomogeneities in the magnetic field due to a material with a magnetic susceptibility different than that of tissue [3-6]. Initial attempts to position and visualize endovascular devices in MR imaging were based on passive susceptibility artifacts produced by the device when exposed to the MR field.
U.S. Pat. No. 4,827,931, to Longmore and U.S. Pat. Nos. 5,154,179 and 4,989,608 to Ratner disclose the incorporation of paramagnetic material into endovascular devices to make the devices visible based on magnetic susceptibility imaging. U.S. Pat. No. 5,211,166 to Sepponen similarly discloses the use of surface impregnation of various “relaxants”, including paramagnetic materials and nitrogen radicals, onto surgical instruments to enable their MR identification. However, these patents do not provide for artifact-free MR visibility in the presence of rapidly alternating magnetic fields, such as would be produced during echo-planar MR imaging pulse sequences used in real-time MR guidance of intracranial drug delivery procedures. Nor do these patents teach a method for monitoring with MR-visible catheters the outcomes of therapeutic interventions, such as, for example, drug delivery into brain tissues or cerebral ventricles.
Ultrafast imaging sequences generally have significantly lower spatial resolution than conventional spin-echo sequences. Image distortion may include general signal loss, regional signal loss, general signal enhancement, regional signal enhancement, and increased background noise. The magnetic susceptibility artifact produced by the device should be small enough not to obscure surrounding anatomy, or mask low-threshold physiological events that have an MR signature, and thereby compromise the physician's ability to perform the intervention. These relationships will be in part dependent upon the combined or comparative properties of the device, the particular drug, and the surrounding environment (e.g., tissue). No additional processing or hardware is required with passive tracking techniques. However, an additional limitation with passive tracking is that quantitative information about the catheter position or orientation is not obtained. This inhibits automatic scan plane prescription and the catheter must be manually kept within the scan plane.
U.S. Pat. No. 5,470,307 to Lindall discloses a low-profile catheter system with an exposed MR-visible coating containing a therapeutic drug agent, which can be selectively released at remote tissue sites by activation of a photosensitive chemical linker. However, in common with other currently used endovascular access devices, such as catheters, microcatheters, and guidewires, the catheter tip is difficult to see on MRI because of inadequate contrast with respect to surrounding tissues and structures. This makes accurate localization difficult and degrades the quality of the diagnostic information obtained from the image. Also, the mere observation of the location of the catheter in the drug delivery system does not reliably or consistently identify the position, movement and/or efficient delivery of drugs provided through the system. Thus, one objective of this invention is to provide for an MR-compatible and visible device that significantly improves the efficacy and safety of drug delivery at various tissue locations using MR guidance.
An improved method for passive MR visualization of implantable medical devices is disclosed in U.S. Pat. No. 5,782,764 to Werne. This invention minimizes MR susceptibility artifacts and controls eddy currents in the electromagnetic scattering environment, so that a bright “halo” artifact is created to outline the device in its approximately true size, shape, and position. In the method of the invention disclosed by Werne, an ultra thin coating of conductive material comprising 1-10% of the theoretical skin depth of the material being imaged is applied, wherein the susceptibility artifact due to the metal is negligible due to the low material mass. At the same time, the eddy currents are limited due to the ultra-thin conductor coating on the device.
A similar method employing a nitinol wire with Teflon coat in combination with extremely thin wires of a stainless steel alloy included between the nitinol wire and Teflon coat, has been reported in the medical literature by Frahm et al., Proc. ISMRM, 3, 1997, p. 1931.
Active catheter tracking requires the reception of the MR signal by the catheter through a receive coil located on a device that is coupled via a cable to the input port of an MR scanner. Henceforth, a receive coil located on a device and coupled via a cable to the scanner will be referred to as a microcoil. Most active tracking techniques project the magnitude sensitivity pattern of small microcoils located on the catheter onto three orthogonal axes. The location of the micro-coils can then be determined by identifying the peaks of the projections [7-9].
There is, however, a weakness with this approach in that the peaks of the projections do not necessarily correspond to the center of the micro-coil. The magnitude sensitivity profile of a coil also changes with different coil orientations. This makes peak finding through curve fitting difficult. The method is also inherently susceptible to noise; high-resolution scans are needed; and it is not possible to obtain orientation information from the magnitude projections of a single coil [10].
Exemplary of methods for active MR visualization of interventional medical devices is U.S. Pat. No. 5,211,165 to Dumoulin et al., which discloses an MR tracking and localizing system for a catheter based on transmit/receive microcoils positioned near the end of the catheter. Applications of such catheter-based devices in endovascular and endoscopic imaging have been described in the medical literature, for example, Hurst et al., Mag. Res. Med., 24, 1992, pp. 343-357, Kantor et al., Circ. Res., 55, 1984, pp. 55-60; Kandarpa et al., Radiology, 181, 1991, pp. 99; Bomert et al., Proc. ISMRM, 3, 1997, p. 1925; Coutts et al., Proc. ISMRM, 3, 1997, p. 1924; Wendt et al., Proc, ISMRM, 3, 1997, p. 1926; Langsaeter et al., Proc. ISMRM, 3, 1997, p. 1929; Zimmerman et al., Proc. ISMRM, 3, 1997, p. 1930; and, Ladd et al., Proc. ISMRM, 3, 1997, p. 1937.
Various imaging coils for interventional MRI are known in the art. U.S. Pat. No. 5,738,632 to Karasawa discloses an endoscope/rigidoscope with MRI coils located in the distal section of the device. U.S. Pat. No. 5,699,801 to Atalar et al describes a loop antenna for interventional MRI and spectroscopy applications. U.S. Pat. No. 5,348,010 to Schnall et al. discloses an inflatable MRI receiver coil employing a balloon.
U.S. Pat. No. 5,271,400 describes a tracking system for the position and orientation of an invasive device within a patient, wherein the device includes a receiver coil and an MR active sample. The receiver picks up magnetic resonance signals generated by the sample. The frequencies are proportional to the location of the coil along the applied field gradients, since the signals are received in the presence of these magnetic field gradients. The system is designed to enable location of the invasive device and enhanced imaging of a region around the invasive device.
U.S. Pat. Nos. 6,587,706 and 6,560,475 to Viswanathan disclose microcoils which can be used in medical devices to enhance RF response signals and to create fields to enhance imaging capability in MRI imaging systems. The microcoil design includes at least one pair of radially opposed microcoils, each microcoil having an outside microcoil diameter of 6 mm or less, individual windings of each microcoil together defining a geometric plane for each microcoil, and the plane of each microcoil being parallel to the plane of another microcoil in the pair of radially opposed microcoils.
U.S. Pat. No. 6,549,800 to Atalar et al discloses methods for in vivo magnetic resonance imaging, wherein MRI probes are adapted for insertion into a plurality of body orifices in order to evaluate the anatomy of proximate anatomic structures, to diagnose abnormalities thereof and to treat the diagnosed abnormalities. MRI probes are described that are suitable for use in the mediastinum, in the pancreaticohepaticobiliary system, in the tracheobronchopulmonary system, in the head and neck, in the genitourinary system, the gastrointestinal system, the vascular system, and in the evaluation, diagnosis and treatment of internal fluid collections.
U.S. Pat. No. 6,061,587 to Kucharczyk et al. discloses an apparatus and method for targeted drug delivery into a living patient using catheter-based microcoils and magnetic resonance (MR) imaging. The apparatus and method uses MRI to track the location of drug delivery and estimating the rate of drug delivery.
A different approach for remote sensing of location is disclosed by U.S. Pat. No. 5,042,486 to Pfeiler et al. and by U.S. Pat. No. 5,391,199 to Ben Haim. These technologies are based on generating weak radio-frequency signals from three different transmitters, receiving the signals through an RF antenna inside the device, and calculating the distances from the transmitters, which define the spatial location of the device. However, the application of this technology to MRI is problematic due to the simultaneous use of RF signals by the MR scanning. Potential difficulties are the heating of the receiving antenna in the device by the high amplitude excitation RF transmissions of the MRI scanner and artifacts in the MR image.
U.S. Pat. Nos. 5,271,400 and 5,211,165 to Dumoulin et al. disclose a tracking system employing magnetic resonance signals to monitor the position (since mentioned below that orientation information is not available) of a device within a human body. The device disclosed by Dumoulin's inventions have an MR-active sample and a receiver coil which is sensitive to MR signals generated by the MR-active sample. These signals are detected in the presence of MR field gradients and thus have frequencies which are substantially proportional to the location of the coil along the direction of the applied gradient. Signals are detected by sequentially applied, mutually orthogonal magnetic gradients to determine the device's position in several dimensions. The position of the device as determined by the tracking system is superimposed upon independently acquired medical diagnostic images. However, this method cannot directly determine the orientation of the device, may be subject to heating of the coil, and requires time to implement that reduces the temporal resolution available for repeated MRI acquisitions.
Although the patented inventions referenced above provide useful technological advances in the field of image-guided interventions, each invention also has significant inherent limitations. Unlike the present invention, which is based on phase information, the prior art references are based on magnitude information. Phase information can track an interventional device more accurately than magnitude sensitivity information because phase information is more spatially varying than magnitude projections. The present invention also provides notable advantages over the prior art by enabling the position and orientation of a catheter tip to be reliably tracked using low resolution MR scans for real-time interventional MRI applications. The prior art methods are also inherently susceptible to noise; high-resolution scans are needed; and it is not possible to obtain orientation information from the magnitude projections of a single coil [10].